Micro-alloyed steels
 |
Huchtemann, Bernd; Engineer, Serosh; Schuler, Volker; |
The invention relates to a micro-alloyed pearlitic steels contain 0.20 to 0.50% C, 0.40 to 1.0% Si, 0,80 to 1,80% Mn, 0.008 to 0.2% S, 0 to 0.7% Cr, 0 to 0.1% Al, 0 to 0.04% N, 0.01 to 0.05% Ti, remainder iron and impurities resulting from the melting process, with mixed sulphides (manganese, titanium carbonitrides, and the like) precipitated on the grain boundaries. The steel may preferably contain an additional content of up to 0.20% V and/or up to 0.10% Nb.
The invention relates to micro-alloyed pearlitic steels for formed parts preferably obtained by forging at high temperatures.
(Percentages indicated in the text are mass %). The effect of a fine titanium nitride dispersion of low concentration on the resistance against grain growth is well known (C. J. Cuddy, J. C. Raley: "Austenite Grain Coarsening in Microalloyed Steels", Metallurgical Trans. A. Vol. 14 A, October 1983, P. 1989-1995). It is, however, hardly possible to make large-scale technical use of this effect, since at usual solidification speeds the temperature range between liquidus and solidus temperature is passed too slowly.
Object of the invention is to provide steels with a high fine grain resistance at temperatures of up to 1300.degree. C. having high strength and toughness also in large-scale production.
Referring to the drawings:
FIG. 1 shows the properties of steels with about 0.25% C and 1.5% Mn and additions of Nb, Nb+V and V+Ti. The dimension is 50 mm .phi. and the sample position is 8 mm beneath the surface.
FIG. 2 is the microstructure of Steel G in the core of 50 mm .phi. after treatment at 1300.degree. C. and 0.5 h/air.
FIG. 3 shows the grain size of the steels of claim 4, without and with titanium. The dimension is 50 mm diameter.
The micro-alloyed pearlitic steels according to the invention contain 0.20 to 0.50% C, 0.40 to 1.0% Si, 0,80 to 1,80% Mn, 0.008 to 0.2% S, 0 to 0.7% Cr, 0 to 0.1% Al, 0 to 0.04% N, 0.01 to 0.05% Ti, remainder iron and impurities resulting from the melting process, with mixed sulphides (manganese, titanium carbonitrides, and the like) precipitated at the grain boundaries. The steel may preferably contain an additional content of up to 0.20% V and/or up to 0.10% Nb.
Suchlike steels can solidify at a solidification speed within the range of 3 to 25 mm/min whereby finely dispersed sulphides precipitate between liquidus and solidus temperatures on grain boundaries.
The fine precipitates in this phase are responsible for the high fine grain resistance and the resulting combination of high strength and high toughness (FIG. 1).
A preferred steel composition is 0.20 to 0.35% C, 0.50 to 0.80% Si, 1.00 to 1.70% Mn, 0.01 to 0.09% S, 0.20 to 0.50% Cr, 0.015 to 0.06% Al, 0.015 to 0.030% N, 0.05 to 0.15% V and/or 0.02 to 0.10% Nb, 0.01 to 0.04% Ti, remainder iron including impurities resulting from the melting process.
It was found that only steels of the claimed composition have a high fine grain resistance at hot forming or annealing temperatures of up to 1300.degree. C. (see FIGS. 1 and 2). The examples given in tables 2 and 3 clearly show that of all steels (composition see table 1) only the steels F and G which are subject matter of the invention present the excellent combination of high strength and high toughness. These steels have a tensile strength of at least 800 N/mm.sup.2, a 0.2% yield strength of at least 550 N/mm.sup.2, at least 15% elongation at rupture (l.sub.o =5 d.sub.o) and at least 45% reduction area. The notch impact values (determined on DVM samples) at room temperature are at least 35 Joule. Although the comparison steels A to E without titanium also have high strength, see table 2, they present an unsufficient toughness of less than 30 Joule, see table 3.
Steels of the following range are also preferred:
0.35 to 0.45% C, 0.5 to 0.8% Si, 1.0 to 1.7% Mn, 0.01 to 0.09% S, 0.2 to 0.5% Cr, 0.015 to 0.06% Al, 0.015 to 0.030% N, 0.05 to 0.15% V and/or 0.02 to 0.10% Nb, 0.01 to 0.04% Ti, remainder iron including impurities resulting from the melting process.
It was found that these steels also have a high fine grain resistance at hot forming or annealing temperatures of up to 1300.degree. C. (see FIG. 3).
The examples given in tables 4 and 6 (compositions see table 4) illustrate that only the steel I and J which are subject matter of the invention present the excellent combination of high strength and high toughness. These steels have a tensile strength of at least 850 N/mm.sup.2, a 0.2% yield strength of at least 600 N/mm.sup.2, at least 12% rupture elongation (l.sub.o =5 d.sub.o) and at least 40% reduction area rupture and a notch impact work on DVM samples at room temperature of at least 30 Joule. Comparison steel 4 without titanium showed with less than 22 Joule unsufficient toughness.
This combination of an even higher strength and good toughness of at least 30 Joule is unusual for micro-alloyed pearlitic steels.
Therefore, the steels which are subject matter of the invention are excellently suitable to be used as automobile structural parts.
Properties over the whole cross section until the core Dimensions of the steels: 50 mm diameter
TABLE 1
______________________________________
chem. composition in mass %
Steel
C Si Mn P S Cr Al N Nb V Ti
______________________________________
A .26 .54 1.37 .005 .014 .40 .043 .019 -- -- --
B .24 .51 1.57 .012 .031 .58 .016 .022 .04 -- --
C .26 .61 1.48 .005 .009 .31 .014 .031 .05 -- --
D .25 .53 1.34 .005 .013 .37 .032 .020 -- .09 --
E .25 .64 1.44 .006 .024 .35 .012 .016 .04 .05 --
F.sup.(1)
.24 .65 1.59 .007 .028 .37 .022 .021 .02 .09 .016
G.sup.(1)
.27 .66 1.43 .014 .036 .10 .024 .017 -- .10 .018
______________________________________
TABLE 2
______________________________________
mechan. post-treatment properties
1250.degree. C. 2 h/air
1300.degree. C. 2 h/air
R.sub.p 0,2
R.sub.m A.sub.5
Z R.sub.p 0.2
R.sub.m
A.sub.5
Z
Steel
N/mm.sup.2
N/mm.sup.2
% % N/mm.sup.2
N/mm.sup.2
% %
______________________________________
A 574 841 15.3 41
558 830 15.7 44
B 523 857 17.5 38
539 889 16.3 28
C 504 862 19.7 36
509 857 18.3 33
D 665 894 17.3 44
622 883 15.0 49
E 610 899 12.0 36
657 832 14.0 41
F.sup.(1)
608 859 16.7 62
565 809 16.7 56
G.sup.(1)
571 811 21.2 64 550 809 22.0 66
562 806 22.0 64 567 806 22.0 64
______________________________________
TABLE 3
______________________________________
notch bar impact work on DVM
samples after treatment
1250.degree. C. 2 h/air
1300.degree. C. 2 h/air
Steel A.sub.v in Joule
A.sub.v in Joule
______________________________________
A 9 13 20 27
B 14 16 18 24
C 17 19 19 23
D 17 19 20 25
E 7 8 12 13
F.sup.(1)
35 38 47 60
G.sup.(1)
55 57 59 64 39 44 46 53
______________________________________
.sup.(1) according to the invention
Properties over the whole cross section until the core Dimensions of the steels: 50 mm diameter
TABLE 4
______________________________________
chem. composition in mass %
Steel
C Si Mn P S Cr Al N Nb V Ti
______________________________________
H .35 .66 1.35 .008 .043 .23 .025 .015 .02 .09 --
I.sup.(2)
.35 .66 1.36 .008 .045 .23 .023 .016 .02 .09 .013
J.sup.(2)
.34 .75 1.33 .006 .055 .27 .008 .018 -- .09 .021
TABLE 5
______________________________________
mechan. post-treatment properties
1250.degree. C. 2 h/air
1300.degree. C. 2 h/air
R.sub.p 0,2
R.sub.m A.sub.5
Z R.sub. p 0,2
R.sub.m
A.sub.5
Z
Steel
N/mm.sup.2
N/mm.sup.2
% % N/mm.sup.2
N/mm.sup.2
% %
______________________________________
H 628 923 15.8 38 651 944 12.8 22
622 921 16.2 39 624 921 11.4 20
I.sup.(2)
606 889 20.8 54 609 894 20.8 54
605 886 20.6 56 603 896 20.2 51
J.sup.(2)
612 873 19.8 56 617 851 20.0 56
602 865 20.0 56 605 859 24.6 56
______________________________________
TABLE 6
______________________________________
notch bar impact work on DVM
samples after treatment
1250.degree. C. 2 h/air
1300.degree. C. 2 h/air
Steel A.sub.v in Joule
A.sub.v in Joule
______________________________________
H 13 15 17 18 13 16 20 21
I.sup.(2)
30 35 38 40 30 31 34 37
J.sup.(2)
40 47 49 59 35 38 42 44
______________________________________
.sup.(2) according to the invention
|
